METAMORPHIC CORE COMPLEXES AT CRUSTAL THRUST RAMPS: A TEMPLATE FOR TECTONIC INHERITANCE (Invited Presentation)

The Cordilleran metamorphic core complexes (mccs) of the Sevier-Laramide hinterland of the western United States provide a case study of tectonic inheritance and reactivation. Here we focus on examples from the Sevier-Laramide hinterland where thick passive margin stratigraphy is involved, but some of the principles apply to mccs developed in the craton of the Mojave and Sonoran deserts. While it is commonly recognized that the mccs are localized in sites of significant Mesozoic crustal thickening, we emphasize their location at major hinterland crustal ramps. At these ramps, a reduction in flexural rigidity due to elevated heat flow and a multilayer rheology led to underthrusting or footwall collapse, resulting in large panels of rheologically stratified crust with a significant dip. The deepest exhumed levels of these underthrust panels record metamorphic pressures of 8-10 kbars. The highest-grade rocks were brought to the surface during multiple phases of exhumation, including combinations of Late Cretaceous, Eocene, Oligocene, and Miocene extension, during repeated episodes of shear zone reactivation. Diapirism played only a minor role in exhumation and locally warped the extensional shear zones, and is most evident where synextensional plutonism occurred. Recognized processes for strain localization during episodic deformation played significant roles in promoting reactivation, including chemical softening – both hydrolytic and reaction – grain-size reduction leading to grain-size sensitive creep, and fabric and geometric softening by development of dipping planar anisotropies. The dip of lithologically-controlled rheological stratification due to underthrusting and the prior anisotropic deformation fabrics have influenced the resulting geometries of many extensional detachment fault systems. Multiple episodes of extensional layer-parallel shear led to collapsing of isograds, as isograds initially cut across stratigraphy in the dipping panels. We discuss examples of tectonic inheritance and shear zone reactivation from the Raft River-Albion-Grouse Creek, Ruby Mountain-East Humboldt Range, Funeral Mountains, northern Panamint Range, and Mesquite Mountains.